A far as I understand, you have a multipages A4 sized pdf and you need to Impose conveniently these A4 pages over an A1 sized page in order to fitting and cover the whole A1 area of A1 sheet with a4 pages
since an A1 portrait oriented ISO page has dimensions 59.4 x 84.1 cm
and an A4 portrait oriented ISO page has dimensions 21 x 29.7 cm
it follows that into an A1 landscape ISO page 84.1 x 59.4 cm we can put (disposed into two rows) 4 A4 portrait pages for row

in our case, having 12 A4 pages,. last 4 columns are left blank
this is called imposition and an easy, cross-platform way to perform this task is using the excellent
you can use, on this last file,
Multivalent.jar (last free version with pdf tools included)
with this syntax:
java -cp /path/to.../Multivalent.jar tool.pdf.Impose -dim 4x2 -paper 84.1x59.4cm file.pdf
and the result will be this:


but if you want all 12 months (pages) into one A1 page you can:
place these 12 pages on a 6x2 grid that fits in A1 landscape bigger dimension (84.1); so, you need to divide this dimension by six (to simplify you'll divide 84 cm by 6 omitting the decimal
you'll have a width of 14 cm
now, since if you resize the page to fit, the dimensions must to have the same original ratio of original A4 page, you'll calculate this ratio dividing 29.7/21 = 1.41428571428571
now you multiply 14 for 1.41428571428571 and you'll have another dimension (the height) = 19.8 cm having the same geometry of A4 original page, since you have proportionally scaled with this syntax:
java -cp /path/to.../Multivalent.jar tool.pdf.Impose -dim 1x1 -paper 14x19.8cm GJP_Koledar_2013_2014_publikacija.pdf
and rename the resulting pdf file 14x19.8.pdf

now, on resulting resized multipage pdf you'll perform this IMPOSITION
java -cp /path/to../Multivalent.jar tool.pdf.Impose -dim 6x2 -paper 84.1x39.6 cm 14x19.8.pdf
with these results:

then, as final stage you'll center this grid of 6 columns x 2rows measuring 84.1x39.6 cm in an A1 landscape ISO page
with the help of jpdftweak
using these settings: (center instead of enlarge in page size tab)

final results:
